1. Field of the Invention
The present invention relates to a calcium sulfide oxidation method and apparatus for oxidizing calcium sulfide (CaS) generated at a power plant, etc., to thereby obtain calcium sulfate (CaSO.sub.4).
2. Description of the Prior Art
One example of a prior art oxidation apparatus for oxidizing CaS generated at a power plant, etc., into CaSO.sub.4 is shown in FIG. 10. In FIG. 10, numeral 1 designates an oxidation apparatus, numeral 1A designates a fluidized bed formed therein and numeral 1A designates a plenum. Numeral 6 designates a heat exchanger disposed in the oxidation apparatus 1, numeral 7 designates a cyclone and numeral 8 designates a particle distributor.
Numeral 9 designates a distributor plate disposed at a bottom portion of the oxidation apparatus 1. On this distributor plate 9, the fluidized bed 1A is formed, and limestone particles 100 containing char and CaS are supplied into the fluidized bed 1A through a nozzle 2. A mixture gas 101 of oxygen, steam and nitrogen is supplied into the plenum 1D through a nozzle 3. The mixture gas 101 is supplied into the fluidized bed IA via the distributor plate 9 to vigorously effect a mixture combustion of the particles 100 in the fluidized bed 1A.
The oxygen concentration of a combustion gas 103 coming out of the fluidized bed 1A is set to 3 to 4% or more. Unless the oxygen concentration of 3 to 4% or more is maintained, it will be difficult to burn the char constantly. In the fluidized bed 1A, there occurs a reaction of CaS+2O.sub.2.fwdarw.CaSO.sub.4 between CaS and oxygen in the gas. While a large proportion of CaS is converted to CaSO.sub.4 as a whole in the fluidized bed 1A, CaS remains still within the particles.
The heat exchanger 6 is disposed in the fluidized bed 1A so that heat of the particles in the fluidized bed 1A is collected and a heating medium fluid 107 flowing in the heat exchanger 6 is heated. Combustion gas 108 coming out of the oxidation apparatus 1 enters the cyclone 7 to be separated into a dedusted combustion gas 109 and collected particles 110. The 30 collected particles 110 are distributed by the particle distributor 8 into fine powder particles 111 to be extracted outside the system and coarse particles 112 to be returned into the fluidized bed 1A.
The coarse particles 112 are supplied into the fluidized bed 1A via a nozzle 5.
Coarse particles 102, an ash content of the char, which are not pulverized in the fluidized bed 1A, but remain there so as not to be elutriated by the gas 103, are extracted outside the system via a nozzle 4 which is fitted to the distributor plate 9.
In the prior art apparatus as described above, there is contained in the particles 111 and 102 extracted outside the system a high concentration of CaS which has not been converted into CaSO.sub.4. This high concentration CaS contained in the particles 111 and 102 is gradually decomposed in the air to generate H.sub.2 S, which results in the problem of an unfavorable influence being given to the environment.
Two reasons are considered why CaS remains in the particles discharged from the prior art oxidation apparatus. Firstly, CaSO.sub.4 generated on a particle surface at an initial stage of reaction forms a dense shell, so that oxygen is not supplied into the interior of the particle and CaS therein cannot react with oxygen. CaSO.sub.4, as compared with CaS, has a molecular volume of 1.8 times as larger, and as the reaction proceeds from CaS into CaSO.sub.4, gas diffusion pores existing in the particle clog, and oxygen cannot be supplied into the interior of the particle.
Secondly, a fine powder begins to entrain from the fluidized bed before ensuring sufficient reaction time required for complete oxidation of CaS, is discharged outside the oxidation apparatus as CaS contained in the fine powder, and is not yet completely oxidized.
Also, in case the fuel supply rate varies, because it is necessary to maintain the temperature and gas flow velocity in the fluidized bed within an appropriate range, it is preferable to change the heat transfer rate of heat transferred to the heating medium through the heat exchanger in the fluidized bed corresponding to the fuel supply rate.
In the prior art, however, it has been difficult to greatly change the heat transfer rate unless the height of the fluidized bed is changed. Further, in changing the fluidized bed height, it is necessary to put in or take out fluid medium to or from the fluidized bed, which work requires a great amount of time, and there has been a problem in that variations in the fuel supply rate cannot be followed well.